1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus.
2. Description of the Related Art
Conventional image forming apparatuses that use an intermediate transfer method are well known. Such an image forming apparatus includes an image carrier, an intermediate transfer member, a primary transfer unit that transfers a toner image from the image carrier to the intermediate transfer member (primary transfer), and a secondary transfer unit that transfers the toner image from the intermediate transfer member to a transfer sheet (secondary transfer). A photoconductor is typically used as the image carrier for carrying a toner image according to image data. The intermediate transfer member is an endless belt wound over a plurality of rollers. An electric field is required both in the primary transfer unit and the secondary transfer unit to effect the primary transfer and the secondary transfer, respectively. The primary transfer unit and the secondary transfer unit must accurately, reliably and efficiently transfer the toner image.
To reliably perform the primary transfer, it is imperative to maintain a contact pressure between the photoconductor and the intermediate transfer belt within a certain range. If the contact pressure is too low, even transfer cannot be attained, i.e., the resultant toner image has an uneven density. If the contact pressure is too high, a hollow portion (particularly in lines and characters) appears in the resultant toner image (hereinafter, “hollow defect”). To remedy this problem, Japanese Patent Laid-Open Publication No. 2000-162899 and Japanese Patent Laid-Open Publication No. 2001-235946 disclose a technology for controlling the contact pressure range between the photoconductor and the intermediate transfer belt.
However, the ideal contact pressure range for realizing reliable transfer varies according to the thickness of the intermediate transfer belt and the average roundness of the toner particles. Thus, reliable transfer cannot always be attained with the same contact pressure range.